Another commander for the group, that is known to host foreign militants in the lawless tribal regions of Pakistan and Afghanistan, said that they were bound to help because Syrian fighters had supported them in Pakistan.

“We have established our own camps in Syria, but some of our people go and then return after spending some time fighting there,” he added.

The militant group is led by Hakimullah Mahsud – who the FBI offered a $5 million reward for on their Most Wanted Terrorist list.

However, senior Pakistani government and military authorities challenged the group's claims.

"How can they claim to be sending fighters to Syria when they failed to resist Pakistani security forces and surrendered their strongholds in the tribal areas," a senior military official in Peshawar said.

Their claims come as an increasing number of foreign fighters enter the two-year-old conflict that has killed more than 90,000 people, according to the United Nations.

Guerrillas from the Lebanese Shiite group Hezbollah have recently helped forces loyal to Assad recapture important regions in central Syria.

Troops backed by tanks and artillery also moved into a rebel-held district of Damascus on Monday, stepping up efforts to drive opposition fighters from the capital and build on battlefield gains elsewhere in the country, a rebel commander told NBC News.